Plasma processing apparatuses are in practical use. A plasma processing apparatus uses plasma to perform varying processes, including forming a thin film, edging, and reforming a substrate for e.g. a semiconductor wafer, a flat-panel display, and so forth. A typical operation performed by such apparatus is to introduce processing gas into the chamber, then apply a high-frequency electric power to the processing gas inside the chamber to produce plasma, and process substrate with the plasma. Since plasma is unstable, a difficulty is encountered in a measurement to grasp in what condition the plasma is—among others, in such processes as treat various elements, like processes in semiconductor manufacturing.
Where the high-frequency electricity is applied inside a chamber in a plasma processing apparatus, abnormal discharges, exemplified by an electric arch, may be caused due to various factors. Such abnormal discharge causes damage to the substrate and components disposed inside the chamber. The damage is, more specifically, cracks, notches, or the like caused on the surface of a semiconductor wafer configured for use as a substrate of microcircuit, or burning-out of components. The abnormal plasma discharge also causes detach of deposits stuck to a component inside the chamber, such as an upper electrode, and produces particles inside the chamber.
Earlier detection of abnormal discharge and subsequent stopping of the operation are required in the plasma processing apparatus, in order to preclude damage to the semiconductor wafer and the components, and preclude the production of particles which the abnormal plasma discharge would cause. For readily detecting the abnormal discharge, several methods are conventionally proposed: one is by monitoring a current value supplied from a power-feeding rod connected to an electrode inside the chamber, and another is by monitoring reflected waves of a high-frequency voltage reflected from such electrode. However, either of the above is of poor sensitivity and, among others, incapable of detecting minimal abnormal discharge.
Thus, a method of detecting a phenomenon of AE (Acoustic Emission) occurring during the abnormal discharge has been developed in recent years. In this method, an ultrasonic sensor is used to detect ultrasonic waves based on the emission of energy during the abnormal discharge.
Unexamined Japanese Patent Application KOKAI Publication No. 2003-100714 (hereinafter referred to as Patent Literature 1) describes examples of apparatuses that use the aforementioned method. One of these is a device that is equipped with a plurality of ultrasonic sensors disposed on an outside wall of the chamber and configured to detect ultrasonic waves caused by emission of energy during abnormal discharge by using such sensors, and another is a device that is equipped with a plurality of acoustic probes which are disposed in contact with a susceptor used as a mounting table on which a semiconductor wafer is mounted or with a focus ring disposed in the vicinity of the mounted semiconductor wafer, and an ultrasonic detector that detects ultrasonic waves propagated from the such acoustic probes, to thereby detect the above-mentioned ultrasonic waves.
The ultrasonic sensors detect as signals not only ultrasonic waves caused by the emission of energy during abnormal discharge, but also noise caused by a mechanical vibration due to opening/closing of a gate valve of the plasma processing apparatus. Therefore, it is necessary to distinguish which of ultrasonic waves caused by the abnormal discharge and the noise due to mechanical vibration, the ultrasonic sensors have detected.
As an art to detect the abnormal plasma discharge, for example, a device disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2003-173896 (hereinafter referred to as Patent Literature 2), comprises acoustic sensors provided in the plasma processing apparatus, and a processing unit of the abnormal discharge detecting device of the plasma processing apparatus comprising at least of a processing unit that processes the detection output of the ultrasonic waves conducted by the ultrasonic sensors to detect the abnormal discharge, which includes at least 2 filters of a first filter relatively transmitting only signals on low-frequency and a second filter relatively transmitting only signals on high-frequency.
Additionally, a device disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2006-128304 (hereinafter referred to as Patent Literature 3) comprises acoustic sensors and potential probes in the plasma processing apparatus, and detects a potential variation on the potential probes, and further detects the ultrasonic waves in the ultrasonic sensors. If it is judged that the ultrasonic waves are detected at the same timing as the potential variation is detected, it will be judged that the abnormal discharge has occurred.
Since the frequency distributions were expected to differ for ultrasonic waves caused by abnormal discharge and for noise due to mechanical vibration, to distinguish between the noise and the ultrasonic waves, it was thought effective to analyze the frequencies of the signals detected by the ultrasonic sensors.
However, as with the Patent Literature 2, in a method that enters time-series data in different band filters and detects the difference of that data by using a ratio of the maximum amplitude for each filter output, the abnormal plasma discharge is not identifiable in the case that the ultrasonic waves caused by abnormal discharge have the same frequency band and amplitude as the sound waves caused by opening/closing valves or as the sound waves in the vicinity of the device.
Experiments conducted by the inventors of the present invention in recent years has proved that the frequency distribution of the ultrasonic waves caused by the abnormal discharge changes according to the position in the plasma processing apparatus where abnormal discharge occurs. In addition, expected is that the frequency distribution of ultrasonic waves caused by abnormal discharge will differ between individual plasma processing apparatuses.
Additionally, in respect of time-series data of the ultrasonic sensors provided in the plasma processing apparatus, an establishment density and a power spectrum may be the same under both of the normal plasma conditions and the abnormal discharge conditions. Thus, the abnormal discharge is not identifiable in conventional methods, because there may be no characteristic difference in the power spectrum even if the frequency power spectrum is estimated and the spectrum shape is compared under the normal conditions and under the abnormal discharge conditions. Thus, the conventional time-series data analysis may not often show the difference between the time-series data clearly and quantitatively.